Some Influences of Stratigraphy and Structure on Reservoir Stress Orientation

Abstract

SPE Members Abstract The orientation of maximum horizontal stress in a reservoir, which strongly influences several drilling, stimulation and production operations, can vary significantly with both depth and structural position. This paper presents field observations, analytical solutions, and numerical model results to illustrate some mechanisms by which stratigraphy and structure act to reorient the direction of maximum stress within anisotropic and heterogeneous reservoir formations. Introduction The orientation of in situ stresses strongly influences several reservoir characteristics related to drilling, stimulation, and production. During drilling operations, for example, the magnitude and orientation of stress determine mud-weight limits required to avoid wellbore collapse or unintentional fracture and lost circulation. Stress orientation is especially important in the design of horizontal completions because stability and fracture of the wellbore varies with the well inclination and azimuth. During production, stress-induced near wellbore damage can cause sanding problems. In naturally or hydraulically fractured reservoirs, effective fracture widths, fluid conductivity, and fracture orientation are directly related to stress direction and magnitude. The resulting permeability anisotropy must be considered when developing dense well spacing designs and line-drive waterflood operations. In many reservoirs the direction of maximum stress can vary significantly with respect to both depth and structural location. This is especially true in reservoirs which are structurally complex, such as in the Rocky Mountain Region, and in tectonically active regions, such as California. Variations in stress orientation are often related to changes in lithology and structure and may arise due to the inherent anisotropic nature of geological materials. While it is common practice to account for reservoir stress magnitude changes with depth and sometimes location throughout a field, relatively little recognition is made regarding variations in reservoir stress orientation and the important implications these variations may have on development strategies. A common practice has been to explain stress anisotropy by imposing a far-field (tectonic) deviatoric stress boundary condition on the reservoir. This assumption is inconsistent with numerous field observations that stress magnitudes vary across lithological boundaries. It is more appropriate to impose tectonic strain or far-field displacement boundary conditions. In doing so, it becomes more clearly apparent that changes in stratigraphic layering, dip, lateral heterogeneity, and local structure act to reorient the principal stress directions. The purpose of this paper is to describe some influences of stratigraphy and structure on stress orientation, and to present analytical and numerical techniques available to estimate changes with depth and structural position. A review is first provided of field evidence which documents variations in maximum stress direction with depth and position around reservoirs. This is followed by a brief description of stress-strain relations for anisotropic geological materials. Some stratigraphic influences on stress orientation are then described analytically and illustrated with a numerical example for stress reorientation caused by changing dip across an unconformity given a uniform farfield displacement boundary condition. Some structural influences are next illustrated with an example of stress variations within an anticlinal structure. Field observations consistent with each of these situations are also presented and discussed. P. 875^